Guides · Jun 13, 2026 · By Fish Tank Volume Calculator Team
How to Calculate Fish Tank Volume (Step by Step)
Learn the simple formula to calculate the volume of any fish tank in litres and gallons, with worked examples for every shape.
Knowing your fish tank volume is the first thing every aquarist should work out. It decides how much water conditioner and medication to dose, which heater and filter to buy, and how many fish you can keep. This guide shows you how to calculate fish tank volume by hand, then convert it to litres and gallons.
The basic fish tank volume formula
For a standard rectangular tank, the formula is simple:
Volume = Length × Width × Height
Measure all three sides in the same unit. Centimetres are easiest, because the result converts cleanly to litres.
Three steps to calculate the volume
- Measure the inside length, width and height. Use the internal measurements, not the outside of the glass, for the most accurate result.
- Multiply the three numbers together. This gives you the volume in cubic units, for example cubic centimetres.
- Convert to litres or gallons. Divide cubic centimetres by 1000 for litres, by 3785 for US gallons, or by 4546 for UK gallons.
Worked example
Take a tank measuring 60 cm long, 30 cm wide and 30 cm high:
60 × 30 × 30 = 54,000 cm³
Divide by 1000 and you get 54 litres, which is about 14.3 US gallons or 11.9 UK gallons.
Tank volume vs real water volume
The figure above is the full capacity. The actual water volume is a little lower, because substrate, rocks and decorations displace water and you always leave a gap at the top. As a rule of thumb, subtract 10 to 15 percent from the calculated volume for a realistic water figure.
Other tank shapes
Not every aquarium is a box. Cylinders, hexagons, corner and bow-front tanks each use their own formula. Rather than work these out by hand, use the fish tank volume calculator on the home page. Pick your shape, enter the dimensions, and it returns the volume in litres, US gallons and UK gallons instantly.
Quick conversions
- Litres to US gallons: multiply by 0.264
- Litres to UK gallons: multiply by 0.220
- Cubic inches to US gallons: divide by 231
Once you know your volume, you can stock and maintain your aquarium with confidence.